This invention relates to a combustion chamber of a gas turbine with starter film cooling of a combustion chamber wall and with several, circularly arranged burners.
The combustion chamber wall encloses a space in which fuel is burnt with the compressed air supplied by the compressor before it is expanded in the turbine to deliver power. The combustion chamber wall must be suitably cooled since the gas temperatures in the combustion chamber generally exceed the melting temperature of the wall material. To ensure longevity, the temperature values must be kept appropriately low. The combustion chamber wall can be equipped with cooling rings (U.S. Pat. No. 4,566,280), effusion holes (U.S. Pat. No. 5,181,379), pinned tiles (EP 1 098 141 A1) or impingement and effusion-cooled tiles (U.S. Pat. No. 5,435,139).
Independently of the cooling method selected, the combustion chamber wall must be protected upstream of the first cooling air inlet, since cooling of the rear side alone is inadequate to keep the temperature level below the applicable limit. Therefore, a so-called starter film is usually applied to the forward part of the combustion chamber wall. This starter film protects the combustion chamber wall until the cooling method actually used has sufficient effect. The air required for this starter film can be supplied from within the space formed by a hood and a base plate or from an annulus between the combustion chamber wall and the combustion chamber casing. The openings in the combustion chamber wall are mostly circular, evenly distributed holes of constant cross-section whose inlet side is neither chamfered nor rounded. The starter film is mainly introduced parallel to and along the combustion chamber wall.
Such a starter film for an effusion-cooled combustion chamber wall is provided in Specification U.S. Pat. No. 5,279,127. However, this Patent Specification only refers to a single-wall design. The gap from which the circumferentially evenly distributed cooling (starter) film discharges is formed by a cooling ring.
In another design known from the state of the art, the air for the starter film is conducted only on one side by way of an element belonging to the combustion chamber wall, while, on the other side, it is confined by a flow surface of the heat shield. The starter film is blown out between the heat shield and the initial portion of the combustion chamber wall to protect this part of the combustion chamber against the hot combustion gases. This is usually accomplished by an evenly distributed number of circular holes arranged on a specific pitch circle on the inlet side, these holes being neither chamfered nor rounded. For uniformity, the individual jets can initially be blown onto the rear of the heat shield. Upon impingement, the jets will cool the heat shield and combine into a homogenous film (starter film) which then flows along the combustion chamber wall. In particular, if effusion cooling is applied for the combustion chamber wall—which can be single-walled or provided with additionally impingement-cooled tiles—a protective cooling film will initially be produced down the stream over a certain distance. Without such a starter film, the initial portion of the combustion chamber wall would not be protected sufficiently.
A disadvantage of the known designs lies in the fact that the starter film is evenly distributed around the entire circumference of the combustion chamber wall. This results in a uniform distribution of the cooling intensity of the starter film. However, since the heat input into the combustion chamber wall increases periodically with each burner and decreases in the spaces between them, a temperature variation will invariably occur in the circumferential direction in the combustion chamber wall. A temperature limit applies to the material which, also at the point of maximum thermal load of the combustion chamber wall, shall not be exceeded. Accordingly, the air quantity of the starter film is controlled by that point on the circumference of the combustion chamber wall which is subject to the highest thermal load, this point being usually situated in the vicinity of the burner axis. However, the quantity of cooling air thus supplied with the starter film to the combustion chamber wall will be excessive in the area between the burners. Consequently, the combustion chamber wall will be overcooled to an unnecessary extent in this area. This non-adaptive cooling method results in pronounced circumferential temperature variations in the combustion chamber wall. These variations, in turn, subject the combustion chamber wall to severe mechanical stresses. These stresses significantly compromise the life of the combustion chamber wall, particularly if effusion cooling is applied.